task(settings): Add marketing section engagement glean event

[dschom]

Because

• We want an event to know if users selected options in the marketing section

This pull request

• Creates a reg_marketing_engage glean event
• Adds it to the signup page
• Adds a standard.marketing label that follows the pattern set by sync.ctws

Issue that this pull request solves

Closes: FXA-8249

Checklist

Put an x in the boxes that apply

• [x] My commit is GPG signed.
• [x] If applicable, I have modified or added tests which pass locally.
• [ ] I have added necessary documentation (if appropriate).
• [ ] I have verified that my changes render correctly in RTL (if appropriate).

Screenshots (Optional)

Please attach the screenshots of the changes made in case of change in user interface.

Other information (Optional)

Note that this glean even has labels that should show up and look like this:

Testing instructions

• Go to sign up
• Enter email
• Enter password and age
• Select some of the options under 'get more from mozilla'
• Observe glean event with labels being emitted

[chenba]

* Adds a standard.marketing label that follows the pattern set by sync.ctws

@ksiegler1 does the 'standard.marketing' name for the marketing email options work for you?

[ksiegler1]

That works for me.

[chenba]

Dan's out for the week. I'll resolve the conflict.